Screw actuator with a fixed nut

ABSTRACT

A screw actuator comprises a housing ( 1 ), a screw mechanism ( 4 ) with a nut ( 6 ) and a screw ( 5 ), as well as a motor ( 7 ) which is drivingly connected to the screw. The screw ( 6 ) is rotatably and translatably supported with respect to the nut ( 5 ), which is fixed with respect to the housing ( 1 ). The screw engages a translatable, non rotatable actuating head ( 29 ) through a swivel connection. The swivel connection comprises a pair of conformed surfaces ( 41, 42 ) having a common rotational axis of symmetry which is aligned with the axis of the screw mechanism.

The invention is related to a screw actuator, comprising a housing, ascrew mechanism with a nut and a screw, as well as a motor which isdrivingly connected to the screw, said screw being rotatably andtranslatably supported with respect to the nut which is fixed withrespect to the housing, and engages a translatable, non rotatableactuating head through a swivel connection.

Such a screw actuator comprises a nut which is fixed with respect to thehousing, that is a nut which is neither rotatable nor translatable.Consequently, the screw should be able to carry out both a rotation anda translation. However, as the actuating head should only be subjectedto translations, a swivel should be present between the screw and theactuating head.

The object of the invention is to provide a screw actuator of this typehaving an improved swivel connection, which also allows for a limitedmisalignment or movements resulting from the inevitable elasticdeformations which develop under the influence of actuating forces.

This object is achieved in that the swivel connection comprises a pairof conformed surfaces having a common rotational axis of symmetry whichis aligned with the axis of the screw mechanism.

The conformed surfaces are in low friction contact with each other,which means that a low friction coating or a lubricant can be providedon their contacting surfaces.

Preferably, the screw comprises a convex surface, and the actuating headcomprises a correspondingly shaped concave surface. The surfaces mayconstitute a ball swivel connection.

In the latter case, the actuating head may tilt with respect to thescrew. Thus, the screw mechanism is protected from bending moments, withrespect to which roller screw, ball screw or friction screw mechanismsare rather vulnerable.

Also, the actuating head may be formed by a piston which is slideablyaccommodated in a cylinder bore in the housing. Although in thisembodiment the piston/cylinder engagement represents a firm support forthe actuating head, nevertheless still some tilting may occur under highactuating forces. This, in such embodiment as well the ball swivelembodiment has favorable effects.

According to an advantageous embodiment, a stack of piezo-electricmaterial layer is enclosed between the surfaces. The actuating forcesexerted by the screw actuator can be measured in a reliable mannerthrough such sensor of piezo-electric material.

The invention will now be described further with reference to anembodiment shown in the FIGURE which shows a cross section.

The FIGURE shows a brake calliper comprising a screw actuator 4 having anut 5, which by means of balls 28 rotatably supports a screw 6. The nut5 and screw 6 have appropriately shaped screw type grooves 31.

An electric motor 7 has a stator 8 connected to the housing 1, as wellas a rotor 9; which is supported by a sleeve 17 rotatably mounted on astationary supporting sleeve 13. The supporting sleeve 13 comprisesraceways 18 for the bearings 19. In the alternative, the sleeve may ofcourse carry the inner rings of separate bearings.

The inner supporting sleeve 13 has an outwardly extending sleeve flange14, which is axially held between a radially inwardly extending supportabutment 11 of the housing 1 and an outwardly extending nut flange 15.

The outwardly extending nut flange 15, as well as the sleeve 13 arefitted within the cylindrical cavity or bore 10 of the housing 1. Thesupport abutment 11 extends radially inwardly with respect to the wallof the cylindrical bore 10.

Non-rotatably held within the bore 10 is a piston 22, within which anactuating head 29 is accommodated. Through a swivel connection 41, 42,the actuating head 29 engages the screw 6 of the screw actuator 4.Separation of the swivel is prevented by means of plate 43.

By activating the motor 7, the rotor 9 drives for rotation the screw 6of the screw actuator 4, via a shaft 26. Said shaft 26 is connected onone side to a reduction gear mechanism 20 which is driven by the rotor 9through the rotor sleeve 17 and an inwardly extending flange 24 thereof.At the other side, the central drive shaft 26 extends into a cylindricalbore 27 of the screw 6. The shaft 26 is non-rotatably coupled to thescrew 6 through a spline groove coupling 60.

As a result, the screw 6 is rotated and moved outwardly, so as to bringbrake pad 3 closer to brake pad 2 for exerting a braking effect on abrake disk (not shown).

As a result of the compressive forces thereby exerted on the nut 5, thesupport sleeve 13 is held firmly clamped against the support abutment 11of the housing 1.

With the aim of accommodating misalignments, which might damage theraceways and balls of the screw actuator, the flange 14 of sleeve 13 aswell as the nut flange 15 may be curved preferably along the surface ofan imaginary sphere, so as to allow some adaptation of the screwactuator 4 together with the rotor 9 and the gear reduction mechanism20.

The swivel part 41, 42 comprises a concave surface 42 of the piston 22,and a convex surface 41 of the actuating head 29. Between these surfaces41, 42, a lubrificant may be present. Moreover, a stack ofpiezo-electric elements 44 may be applied to one or both of thesurfaces, with the aim of measuring actuating forces.

1. A screw actuator, comprising: a housing; a screw mechanism with a nutand a screw; and a motor which is drivingly connected to the screw, saidscrew being rotatably and translatably supported with respect to the nutwhich is fixed with respect to the housing, the screw engaging atranslatable, nonrotatable actuating head through a swivel connectioncomprising a pair of conformed surfaces having a common rotational axisof symmetry which is aligned with an axis of the screw mechanism;wherein a stack of piezo-electric material layers is enclosed betweenthe surfaces, wherein the surfaces are spherical; and wherein the screwcomprises a convex surface, and the actuating head comprises acorrespondingly shaped concave surface, or vice versa.
 2. A screwactuator according to claim 1, wherein the actuating head is formed by apiston which is slideably accommodated in a cylinder bore in thehousing.
 3. A screw actuator according to claim 1, wherein the screwengages a rotatable drive shaft through a ball spline joint, said shaftbeing connected to a rotor of the motor through a gear reductionmechanism.
 4. A screw actuator according to claim 1, wherein the screwmechanism comprises a ball screw, a roller screw or a friction screw. 5.A screw actuator, comprising: a housing; a screw mechanism with a nutand a screw; and a motor which is drivingly connected to the screw, saidscrew being rotatably and translatably supported with respect to the nutwhich is fixed with respect to the housing, the screw engaging atranslatable, nonrotatable actuating head through a swivel connectioncomprising a pair of conformed surfaces having a common rotational axisof symmetry which is aligned with an axis of the screw mechanism;wherein a stack of piezo-electric material layers is enclosed betweenthe surfaces, wherein the surfaces are spherical and constitute a ballswivel connection; and wherein the screw comprises a convex surface, andthe actuating head comprises a correspondingly shaped concave surface,or vice versa.